Forum Replies Created
-
Posts
-
Hi Andrew,
You raise some important and interesting points, possibly worth their own forum threads.
I agree the processing of a spectrum should depend on the scientific use to which it is intended to be used. When submitting to databases though, it is my opinion that it is best not to rectify the spectrum unless necessary. The reason for this is that a researcher can always rectify a spectrum should they wish to do so, though they cannot unrectify it. Note that when a rectified spectrum is submitted to the BAA Spectroscopy Database then the BSS_NORM fits header should be populated.
I have no strong opinion on whether or not to rectify a spectrum when no response correction has been applied.
On the dark vs bias front, I would expect to see differences between dark and bias frames with amateur CCDs. I also use an SXVR-H694 and I see a big difference between my bias and dark frames.
For the following stats I had the CCD set to -20C with 600 second darks, with statistics computed by MaxIm DL from single raw bias and dark frames.
Bias Dark
Mininum 882 1006
Maximum 1315 56522 (due to hot pixels not cosmic ray hits)
Average 1084.580 1133.068
Standard Deviation 15.960 211.396I also advise to flat field as know of a couple of reasons why the response correction from a reference star would not correct for all variations in the CCD.
First, when using a reference star to create an instrumental and atmospheric response curve, then there is a degree of smoothing to remove small scale variations in the curve. This is to give the bulk wavelength dependency. So by its very nature it does not remove the individual pixel to pixel variations.
Second, the target star and reference star on the CCD need to be exactly aligned so that the light path through the spectrograph illuminates the CCD pixels in exactly the same places. This is nearly impossible with a slit spectrograph, but I am not sure if a fibre fed system would get around this.
I am happy to be proven wrong if my understanding is at fault. Also please excuse this lengthy post but I thought it worthwhile for these important topics.
Kind regards,
Andy
Hi Kevin,
I’ve been doing spectroscopy for a couple of years now and I still get excited with each new spectrum. You never know what you will see and this is very true of Be stars. The hydrogen lines can be a complex mix of absorption and emission from the star and disc configurations which change over time.
I found 48 Per in the BeSS database. I’ve put a link below, though it does not look as though the link preserves all of the criteria I entered. You can select the H-beta region by entering 4861 in the wavelength. This shows pervious spectra with various mixes of absorption and emission of this line.
http://arasbeam.free.fr/spip.php?page=beam_splist2&etoile=HD 25940&lang=en
I’m not sure of the effect of rectification on the science value of a spectrum, so I avoid it. The BeSS standard uses the words ‘clearly advised against’ where it calls it ‘continuum normalisation’. For personal interest it is of course fine, and others with more knowledge may be able to advise on its effect. The bit where I am particularly unsure is its effect on line strength measurements, like equivalent width. This may not be big since the effects to the continuum on either side will be scaled by a similar amount, but I’ve never done an experimentation to discover the effect.
Similarly, for personal interest it is fine to use alternative methods to correct the response, but as soon as you want to do any science or submit to a database then a proper response correction is needed.
Best wishes,
Andy
Hi Kevin,
Nice spectrum!
I check, or average out, mechanical shifts in my spectrograph by taking neon calibration frames at the start and end of each stellar spectra. However, I’m using an LHIRES III which is more prone to flexure than the Alpy, plus I frequently stay on the same star for an hour or more. So you may not need to do this for your setup.
When checking a radial velocity, then ISIS has a useful tool in the “Misc” tab for calculating the heliocentric velocity correction, so the rotation of the Earth and its orbit around the Sun (bigger contribution) can be calculated and removed. Then I find SIMBAD is an excellent resource for checking the expected radial velocity of the star.
http://simbad.u-strasbg.fr/simbad/
Best wishes,
Andy
Hi Jack,
The error message or screen shot did not get included in your post.
In general terms, if BASS cannot open it then that implies something went wrong with the calibration process. Before moving to ISIS I carried out bias, dark and flat calibration in MaxImDL, followed by spectrum extraction and wavelength calibration in BASS.
Best wishes,
Andy
Hi Jack,
I would just re-emphasize that the author of BASS is re-writing the calibration part of the software. So expect changes to BASS calibration, and I’m not sure where the deficiencies currently lie in the BASS calibration routines. That is why I was suggesting MaxImDL. Once John has finished with the BASS enhancements I am hopeful that will contain useful features.
I usually take a minimum of 20 calibration frames. This reduces the noise when they are combined, which is an unavoidable element of calibration frames.
-5C is OK but if you can go colder then that will reduce the noise in your spectra. Just emphasizing again that the temperature of your CCD when taking the calibration frames must match the temperature when acquiring spectra. So you might need one set of calibration frames at -5C for your older spectra. Then another set at a cooler temperature if your CCD can go colder and hold a stable temperature to within at most a few tenths of a C.
Best wishes,
Andy
Hi Jack,
I think Robin and Paul have covered this off well, but I’ll add and re-iterate a few points. It is worth noting that this is the same process as is used for deep sky imaging. The main difference is that you can’t use “sky flats” or just any kind of flat lamp. Instead you need a flat lamp that produces a nice smooth continuous spectrum. The LHIRESIII, ALPY, and LISA all have such a flat lamp built in.
I’ll add that your calibration frames need to be taken at the same CCD temperature, and this should be as cold as possible, whilst ensuring that your CCD can easily maintain this temperature. I find that my SXVR-H694 can hold -20C during the Summer as well as the Winter months.
Best practice is to take of the order of 20+ frames of each type. I was actually taking a new set of darks and bias frames last night. Long overdue as over time the performance of a CCD can change. I try to retake darks and bias frames every 6-12 months. Flats I retake about once a month if I don’t touch the spectrograph. If I change anything on the spectrograph then the flats have to be retaken. This includes focusing the spectrograph collimator lens, but not refocusing the telescope.
I know that John Paraskeva is writing some new calibration routines for BASS. Though MaxImDL has perfectly good calibration routines which work fine for spectroscopy.
Best wishes,
Andy
Hi Jack,
First, just to note that PTAREA is not one of the FITS header requirements for BeSS, in case this is why you are asking.
To calculate the area you use the formula for the area of a circle.
Area = pi x radius squared
The Celestron website gives the diameter of your C14 as 355.6mm, so the radius is 177.8mm. Thus the area would be:
3.14159 x 177.82 = 99315mm2
This does not take into account is the central obstruction in your C14. Again the Celestron website gives this as 114mm, so a radius of 57mm. Thus the area of the central obstruction would be:
3.14159 x 572 = 10207mm2
So the actual light collecting area of your C14 will be:
99315 – 10207 = 89108mm2
Cheers,
Andy
Hi Kevin,
I find this depends on the field and the brightness of the star. I typically guide on another star in the field and there are usually several candidates, with my guide images of 5 to 10 seconds. However, this may depend on how good your mount is at tracking.
I use PHD2 and have guided on the target star on occasion. I’ve not found any problem with this, but there does need to be a strong signal outside of the slit. I’ve not looked in any detail as to how this works, whether it needs bimodal guiding or not. If the star is bright enough then I tend to notice it dimming rather than disappearing or splitting in two. This is usually the case when I am taking a spectra of a bright star and so the fainter stars are not easily visible in the guide image.
Cheers,
Andy
Hi Kevin,
For locating targets when polar alignment may be a bit off, I suggest star hopping. Start with a bright star that cannot be mistaken for anything else, then move towards the target star, if necessary locating and centering some other stars of intermediate brightness along the way.
The guide image in spectroscopes tends to be of lower quality due to the transfer optics, but it is a lot better than it used to be. I find that I can sometimes solve my guide images with Astrometry.net, but not always. I’ve also taken to storing a guide image, so that I can always go back and check if the spectrum turns out to be unexpected. Observing variable stars can be a bit easier as there will often be a BAA chart and/or and AAVSO chart. With these I find it straightforward to confirm that I’ve got the right star.
That is a great first spectrum. I agree something does not look right with the continuum, but it can take some trial and error to get the response profile working well. Robin’s suggestion of observing 2 Miles stars, and using one to create the response profile to correct the continuum on the other is very good. Especially if you choose A or B type stars which have simple spectra so it is easier to see what is going on with the response profile. The correspondence between the features in the spectra shows that your wavelength calibration is working well.
Best wishes,
Andy
Hi John,
That is an excellent first spectrum! Anyone starting out in spectroscopy would be very pleased with that. It shows that all of the basics including instrument response have worked as they should. You may as well crop the spectrum at a slightly shorter wavelength as it looks like it is just noise at the far left, but that is a minor point.
To be honest I hardly ever use the “Dispersion” button on tab 5, as the wavelength calibration is usually performed spot on in tabs 3 and 4. I did take a look at the dispersion in tab 5 last night, and it did appear to be populated, but that was in ISIS v5.5.2. It will be interesting to see what others post, but it may be that even if it is not populating you can get by without it, at least for now. What I’m not sure is whether this is an undetected bug, or something to do with how ISIS is used like a setting.
You may also find you get different behaviour with the neon calibration image, if you select ALPY with Calibration module. Though I’m not sure.
Regardless, you are already getting useful spectra with the processing you are doing.
Best wishes,
Andy
Hi John,
That sounds a bit odd. Can I check that the spectrum definitely calibrates successfully. If you hover the mouse over the spectrum plot in tab 5, then do you see sensible wavelength numbers appear for the cursor position in the bottom left of the screen?
Also, you say you are not using the Alpy calibration module. Are you using an A type star, or similar star with prominent hydrogen lines to wavelength calibrate your spectrum? Or are you using a neon lamp source, just not the one that comes with the ALPY, in which case you may be able to select the “ALPY 600 (with calibration module)”.
Cheers,
Andy
The software is in testing and it has not been officially announced. So I’m not sure if it should be discussed publicly on the BAA forum. I’ve emailed the developer to check.
Andy
I did once have condensation forming on my CCD coverslip. This can happen with some CCDs when they are kept in a damp environment.
I now place a silicon desiccant bag in my telescope tube when I’m not using it, which I replace and reactivate every so often. I’ve not had any problems since doing this.
Andy
Hi Kate,
It is one of the idiosyncrasies of ISIS calibration for the Alpy that you have to tweak the value of the pixel size. This goes against common sense as of course the pixel size is fixed.
I think this is because no 2 Alpys are quite identical, and is due to minor differences in focal lengths and chip to grism distances. François Cochard explained this at the BAA workshop far better than I can remember it now, but I remember that tweaking the pixel size fixes the problem.
Best wishes,
Andy
Hi Steve,
A couple of interesting spectra.
With regard to choice of star for the response, as well as choosing an A or B type star the other factor is to have a good professional spectrum to produce the response curve. Stars from the MILES database are the common choice, as this has good coverage across the sky. In a previous post Robin gave a link to a very useful spreadsheet developed by Paolo Berardi which allows you to search for MILES A or B stars for coordinates of interest.
https://britastro.org/node/8152
I think Castor is a reasonable choice, though Robin and David have more experience in choice of good stars for low resolution spectroscopy than I do. Though it is a double star which might make me think twice about using it, it is in the MILES database (HD060179), so that means there should be a good professional reference spectrum.
Cheers,
Andy
Hi Nick,
I have an 55W electric tube heater which I leave on in my observatory. I also have a cold air dehumidifier which I run for a couple of hours after I’ve closed up my observatory. I find things due up during an observing session, and this takes away the moisture. This is particularly important for me as I leave a desktop computer in my observatory.
Andy
Hi Bill,
That is a fantastic looking spectrum with lots of features in it.
If you are able to convert your meteor spectra to 1D FITS format then you will be able to load them into the new BAA Spectroscopy Database. Let me know if you are interested and if I can be of any assistance.
Cheers,
Andy
Very interesting. There is also the SPHERE instrument which is mounted on the VLT.
http://www.eso.org/public/unitedkingdom/teles-instr/vlt/vlt-instr/sphere/
Hi Steve,
I’m not really an Alpy expert but I do agree that looks a bit odd. Was it taken in darkness or daylight? Just in case stray light might have been getting in. Also, do you know if that bright “line” is oversaturated and do you have an approximate idea of its wavelength from your wavelength calibration.
Best wishes,
Andy
Hi Tony,
The new BAA Spectroscopy Database could well be the ideal place to share and compare results, assuming people have reached the stage of producing 1D fits files from ISIS.
I’m just looking at making a final tweak before go-live. This would be to add a “flat field” flag. Though in theory all spectra should be flat fielded before submission, there may be cases where this is not possible, hence why we might want to record that information.
I’ve been busy since the workshop, so I’ve not had a chance to work on this change. If I can’t find the time to do it within the next few days then we’ll probably go-live without it to avoid further delay.
Best wishes,
Andy
